In a modern satellite communications system utilizing frequency reuse, the antenna system is required to be circularly polarized with a high polarization purity oer a broad band-width and, at the same time, must be capable of dual-polarized operation. Microstrip antennas have recently been enjoying growing popularity in various applications due to their inherent features such as low profile, light weight, and small volume. The natural radiation is, however, linearly polarized, and thus the circular polarization technique is needed when the microstrip antenna is to be used in satellite communications.
Circular polarization is achieved by combining two orthogonal linearly polarized waves radiating in phase quadrature. There are currently two commonly used techniques for resonant microstrip radiators: the single feed technique, where asymmetry is introduced into the geometry of the microstrip radiator so that, when excited at a proper point, the antenna radiates two degenerated orthogonal modes with a 90.degree. phase difference; and the dual feed technique, where two separate and spatially orthogonal feeds are excited with a relative phase shift of 90.degree.. For more specific discusson of these techniques, the reader is referred to K. R. Carver and J. W. Mink, "Microstrip Antenna Technology", IEEE Trans. on Antennas and Propagation, Vol. AP-29,. No. 1, January 1981, pp. 1-24. The single feed aproach has the advantage of a simple feed circuit, but suffers from a very narrow useful bandwidth. Examples of the single feed approach include the corner-fed rectangle, the elliptical patch, the square patch with a 45.degree. center slot, the pentagon-shaped patch, and the circular patch with notches or teeth. Such techniques are discussed, for example in M. Hanesishi and S. Yoshida, "A Design of Back-Feed Type Circularly-Polarized Microstrip Dish Antenna Having Symmetrical Perturbation Element by One-Point Feed", Electronics and Communications in Japan, Vol. 64-B, No. 7, 1981, pp. 52-60.
The dual feed approach requires the use of a 90.degree. hybrid or power splitter with unequal lengths of transmission line to provide the necessary phase shift. The usable bandwidth can be very wide if both the microstrip radiator and the feeding network are broadband devices. The technique, however, suffers from poor polarization purity due to the cross-polarized components generated by the asymmetrical feed structure. One method of cancelling the cross-polarized component is to excite the two feeds unequally, as discussed in H. Chen, "STC Microstrip Plannar Array Development", COMSAT Technical Note, 831564/K82, Feb. 15th, 1984. This method will improve one sense of circular polarization at the expense of degrading the other sense of polarization, and, thus, is incapable of dual-polarized operation. The Chen article, which is not prior art as respects the invention, is hereby expressly incorporated by reference herein.
The cross-polarized component can also be eliminated by cutting two notches on the microstrip radiator to compensate for the feed asymmetry as discussed in T. Teshirogi, "Recent Phased Array Work in Japan", ESA/COST 204 Phase-Array Antenna Workshop, Noorwijh, the Netherlands, June 13th, 1983, pp. 37-44. Capable of dual-polarized operation, this approach is, however, empirical and leads to noticeable changes in antenna characteristics such as resonant frequncy, complicating the antenna design procedure.